4.38 Farm Mechanics. 



shortening and a pull and they are arranged in systcnn of 

 pairs designed to produce movements in opposite direc- 

 tions, as illustrated by the biceps and triceps muscles which 

 move the fore arm as represented in Fig. 234. Just how 

 the shortening of the muscle fibers is accomplished under 

 the nervous stimulus sent to it is not clearly understood, 

 but it is known that, while in action, the muscle fibers are 

 in a state of vibration which gives rise to sounds known as 

 the muscular murmur. 



When muscles are in action and are producing mechan- 

 ical movements their temperature changes but little, but 

 if the muscles are helcj in a state of rigid contraction with- 

 out producing motion as the result, then the temperature 

 rises, showing that the energy which normally would be 

 changed into mechanical motion is changed into heat ; this 

 is exactly what occurs in a steam engine. When it is 

 working hard a large portion of the heat energy of the 

 steam is transformed into mechanical work and the heat 

 generated in the fire box thus disappears but the moment 

 the engine is stopped and the steam is held so that it is 

 unable to produce motion of the piston the temperature 

 rapidly rises. 



607. Strength of Muscles The strength of individual 

 muscles is often very great and more than at first seems 

 possible. Taking the case of the biceps and triceps mus- 

 cles in the arm, represented in Fig. 234, it is possible to 

 measure their power with a spring balance. If a loop of 

 strong cord is fastened to each end of a spring balance and 

 the foot put through one, while the hand is put through the 

 other, the strength of the muscle can be measured by lift- 

 ing against the balance, bending the fore arm so as to make 

 a right angle with the arm and holding it horizontal with 

 the elbow against the edge of the desk. 



A man of average strength will exert a pull of 50 pounds 

 in this way ; and as the lever arm upon which the muscle 

 acts is only one-sixth of the length of the weight arm the 

 pull of the muscle must have been 300 pounds. When 



